The present invention relates to a continuous process for preparing lower dialkyl carbonates as main product and alkylene glycol as by-product by transesterification of a cyclic alkylene carbonate (e.g. ethylene carbonate or propylene carbonate) with lower alkyl alcohols in the presence of a catalyst and also the required purification of the dialkyl carbonate in a subsequent process step. To optimize the economics and energy efficiency of the process, additional devices are used for intermediate heating of the liquid streams in the apparatus.
The preparation of dialkyl carbonates from cyclic alkylene carbonate and alkyl alcohol, in which alkylene glycol is simultaneously formed as by-product, is known and has been widely described. In U.S. Pat. No. 6,930,195 B, this catalysed transesterification reaction has been described as a two-stage equilibrium reaction. In the first reaction stage, the cyclic alkylene carbonate reacts with alkyl alcohol to form hydroxyalkyl carbonate as intermediate. The intermediate is then converted by means of alkyl alcohol in the second reaction stage into the products: dialkyl carbonate and alkylene glycol.
For the industrial implementation of the dialkyl carbonate production process, the use of a reactive distillation column (also referred to below as transesterification column), which has been described, inter alia, in EP 530 615 A, EP 569 812 A and EP 1 086 940 A, has been found to be particularly advantageous. In EP 569 812 A, the cyclic alkylene carbonate is fed continuously into the upper part of the transesterification column and the alkyl alcohol containing dialkyl carbonate is fed continuously into the middle or lower part of the transesterification column. In addition, virtually pure alkyl alcohol is introduced below the point of introduction of the alkyl alcohol containing dialkyl carbonate. According to the present invention a substance is referred to as being virtually pure, if the content of impurities is less than 2% by weight, preferably less than 1% by weight. The high boiler mixture which comprises the alkylene glycol produced as a by-product is taken off continuously at the bottom of the transesterification column. The low boiler mixture, which comprises the dialkyl carbonate produced, is taken off at the top of the transesterification column as the dialkyl carbonate/alkyl alcohol mixture and subjected to a further purification step.
The distillation column for purifying the dialkyl carbonate/alkyl alcohol mixture is operated at a higher pressure than that prevailing in the transesterification column, so that a further dialkyl carbonate/alkyl alcohol mixture having a lower dialkyl carbonate content can be taken off at the top of this distillation column. The dialkyl carbonate as main product is obtained in high purity at the bottom of this purification column.
Many factors play an important role in the development of an economically attractive process for preparing dialkyl carbonates. Most literature references are concerned with the reaction parameters such as conversion, selectivity or product purity. The energy efficiency of the process is more rarely addressed (e.g. in EP 569 812 A, JP2003-104937, WO 2007/096340, WO 2007/096343), even though this factor makes a not inconsiderable contribution to the economic attractiveness of the process. In the present invention, measures for increasing the energy efficiency of the process are therefore introduced.
In EP 569 812 A, the energy input in the preparation of the dialkyl carbonate is reduced by many streams in the process not being condensed but being conveyed as gaseous streams.
WO 2007/096340 describes a process in which alkylene carbonate is produced from alkylene oxide and CO2 and the alkylene carbonate is subsequently reacted with alkyl alcohol to form dialkyl carbonate and alkylene glycol, with the mixture formed in the second step, which contains dialkyl carbonate and alkylene glycol, being purified. The reaction to form the alkylene carbonate is exothermic and the corresponding alkylene carbonate product stream is used to heat the dialkyl carbonate/alkylene glycol product stream in the purification.
In WO 2007/096343, the mixture of dialkyl carbonate and alkyl alcohol formed from alkylene carbonate and alkyl alcohol in a transesterification column is purified by means of extractive distillation, with alkylene carbonate serving as extractant. After the dialkyl carbonate has been separated from the extractant by distillation, the hot bottoms from this column, which contain the extractant, is used to heat the alkyl alcohol fed to the transesterification column.
JP 2003-104937 looks at various process variants for working up an ethylene carbonate/ethylene glycol mixture and providing the purified ethylene carbonate for the process for preparing dimethyl carbonate from the point of view of energy consumption, too.
However, none of the abovementioned documents describes processes or procedures by means of which the reaction of alkylene carbonate with alkyl alcohol in the transesterification column can be carried out particularly energy-efficiently while maintaining the quality of the main product (dialkyl carbonate) and of the by-product (alkylene glycol). For this reason, measures for increasing the energy efficiency in this process step are introduced in the present invention.
There was therefore a need for a process which has a higher energy efficiency in the transesterification column while maintaining the same quality of the dialkyl carbonate and the alkylene glycol.